The present invention relates to a light triggered thyristor, and especially to a light guide for transmitting light to a light receiving portion of a thyristor element.
FIG. 1 is a sectional view showing the construction of a conventional light triggered thyristor. In FIG. 1, the numeral 1 designates a light triggered thyristor element having a light receiving portion 1a on the surface thereof. At 2 and 3 are electrodes, at 4 a housing, and at 5 is a light source such as a high powered LED with a directional characteristic. An optical fiber 6 is provided for transmitting light from the light source to the light triggered thyristor, and an internal light guide 7 formed of a bent and transparent glass rod of a constant diameter is fixed on the housing 4 and transmits light received from the optical fiber 6 to the light receiving portion 1a of the light triggered thyristor element 1.
It is necessary, in triggering the light triggered thyristor, to apply light of sufficient energy (several tens of m W) to the light receiving portion 1a. To this end, it is, of course, necessary to provide not only a light source of large power, but an optical fiber having a large numerical aperture and a large diameter as well, to transmit light to the light triggered thyristor with good efficiency. However, the numerical aperture values available are somewhat limited. In an optical fiber of quartz with a low transmission loss and large mechanical strength, the numerical aperture is at the present time limited to the value 0.3. Further, in optical fibers of the multi-component type, which are not as good as quartz fibers in light transmission loss or mechanical strength, the numerical aperture is limited to values from 0.5 to 0.6. Under these circumstances, the diameter of the optical fiber 6 must be increased. On the other hand, it is preferable to have a diameter as small as possible for the light receiving portion 1a of the light triggered thyristor 1, in order to improve the dv/dt capability while maintaining good light-triggering operation sensitivity, since the light-triggering sensitivity is proportional to the product of the dv/dt capability and the area of the light receiving portion 1a.
In a conventional light triggered thyristor having a constant diameter internal light guide, the diameter of the internal light guide 7 has been determined in view of the balance between the output of the light source, the light-triggering sensitivity of the light triggered thyristor element and the dv/dt ratio. Accordingly, the conventional light triggered thyristor undesirably has a disadvantage in that in the case where the light source and the light-triggering sensitivity have been determined, the dv/dt capability cannot be increased to a sufficient value, limiting the area of application of the light triggered thyristor.